This proposal aims to contribute to an understanding of the organization of elementary genetic units in higher organisms and the regulation of the expression of such units in development. The rosy locus of chromosome 3 in Drosophila melanogaster carries the structural information for a peptide which, as a homodimer, functions as xanthine dehydrogenase in this organism. Mapping experiments have served to map the borders of the structural element and variants which map outside and just to the left of the structural element appear to be associated with regulation (i.e., they are alterations in a cis acting control element). The direction of experiments in the current proposal involved selective strategies designed to recover several classes of control variants, (2) analysis of such variants involving immunochemical, thermal, kinetic, cytogenetic, fine structure recombination and developmental studies designed to document these variants, (3) experiments designed to determine direction transcription and translation of this system.